Ultrasonic degassing of molten metals

ABSTRACT

Ultrasonic devices having an integrated purging gas delivery system are described, and such devices can be used to remove dissolved gasses and impurities from molten metals.

REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 61/322,324, filed on Apr. 9, 2010, the disclosure of which isincorporated herein by reference in its entirety.

COPYRIGHTS

All rights, including copyrights, in the material included herein arevested in and the property of the Applicants. The Applicants retain andreserve all rights in the material included herein, and grant permissionto reproduce the material only in connection with reproduction of thegranted patent and for no other purpose.

BACKGROUND

The processing or casting of certain metal articles may require a bathcontaining a molten metal, and this bath of molten metal may bemaintained at a temperature in a range of 700° C. to 1200° C., or more,depending upon the particular metal. Many instruments or devices may beused in the molten metal bath for the production or casting of thedesired metal article. There is a need for these instruments or devicesto better withstand the elevated temperatures encountered in the moltenmetal bath, beneficially having a longer lifetime and limited to noreactivity with the particular molten metal.

Moreover, molten metals may have one or more gasses dissolved in themand/or impurities present in them, and these gasses and/or impuritiesmay negatively impact the final production and casting of the desiredmetal article, and/or the resulting physical properties of the metalarticle itself. Attempts to reduce the amounts of dissolved gasses orimpurities present in molten metal baths have not been completelysuccessful. Accordingly, there is a need for improved methods to removegasses and/or impurities from molten metals.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the detaileddescription. This summary is not intended to identify required oressential features of the claimed subject matter. Nor is this summaryintended to be used to limit the scope of the claimed subject matter.

The present invention is directed to methods for reducing the amount ofa dissolved gas (and/or various impurities) in a molten metal bath(e.g., ultrasonic degassing). In one embodiment, the method may compriseoperating an ultrasonic device in the molten metal bath, and introducinga purging gas into the molten metal bath in close proximity to theultrasonic device. For example, the dissolved gas may comprise hydrogen,the molten metal bath may comprise aluminum or copper (including alloysthereof), and the purging gas may comprise argon and/or nitrogen. Thepurging gas may be added to the molten metal bath within about 50 cm (or25 cm, or 15 cm, or 5 cm, or 2 cm) of the ultrasonic device.

The present invention also discloses ultrasonic devices, and theseultrasonic devices may be used in many different applications, includingultrasonic degassing. As an example, the ultrasonic device may comprisean ultrasonic transducer; an elongated probe comprising a first end anda second end, the first end attached to the ultrasonic transducer andthe second end comprising a tip; and a purging gas delivery system,wherein the purging gas delivery system may comprise a purging gas inletand a purging gas outlet. In some embodiments, the purging gas outletmay be within about 10 cm (or 5 cm, or 1 cm) of the tip of the elongatedprobe, while in other embodiments, the purging gas outlet may be at thetip of the elongated probe. In addition, the ultrasonic device maycomprise multiple probe assemblies and/or multiple probes per ultrasonictransducer.

Both the foregoing summary and the following detailed descriptionprovide examples and are explanatory only. Accordingly, the foregoingsummary and the following detailed description should not be consideredto be restrictive. Further, features or variations may be provided inaddition to those set forth herein. For example, certain embodiments maybe directed to various feature combinations and sub-combinationsdescribed in the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this disclosure, illustrate various embodiments of the presentinvention. In the drawings:

FIG. 1 shows a partial cross-sectional view of an ultrasonic device inan embodiment of the present invention.

FIG. 2 shows a partial cross-sectional view of an ultrasonic device inanother embodiment of the present invention.

FIG. 3 shows a partial cross-sectional view of an ultrasonic device inyet another embodiment of the present invention.

FIG. 4 is a bar graph illustrating the percentage difference in densityfor each of Examples 1-4 as compared to the theoretical density ofaluminum.

FIG. 5 is a bar graph illustrating the hydrogen content in ppm of eachof Examples 1-4.

FIG. 6 is a plot of hydrogen concentration as a function of time forExamples 5-8.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawings.Wherever possible, the same or similar reference numbers are used in thedrawings and the following description to refer to the same or similarelements. While embodiments of the invention may be described,modifications, adaptations, and other implementations are possible. Forexample, substitutions, additions, or modifications may be made to theelements illustrated in the drawings, and the methods described hereinmay be modified by substituting, reordering, or adding stages to thedisclosed methods. Accordingly, the following detailed description doesnot limit the scope of the invention.

The terms “a,” “an,” and “the” are intended to include pluralalternatives, e.g., at least one. For instance, the disclosure of “anultrasonic device,” “an elongated probe,” “a purging gas,” etc., ismeant to encompass one, or combinations of more than one, ultrasonicdevice, elongated probe, purging gas, etc., unless otherwise specified.

All publications and patents mentioned herein are incorporated herein byreference for the purpose of describing and disclosing, for example, theconstructs and methodologies that are described in the publications,which might be used in connection with the presently describedinvention. The publications discussed throughout the text are providedsolely for their disclosure prior to the filing date of the presentapplication. Nothing herein is to be construed as an admission that theinventors are not entitled to antedate such disclosure by virtue ofprior invention.

Applicants disclose several types of ranges in the present invention.When Applicants disclose or claim a range of any type, Applicants'intent is to disclose or claim individually each possible number thatsuch a range could reasonably encompass, including end points of therange as well as any sub-ranges and combinations of sub-rangesencompassed therein. For example, in an embodiment of the invention, thepurging gas may be added to the molten metal bath at a rate in a rangefrom about 1 to about 50 L/min. By a disclosure that the flow rate is ina range from about 1 to about 50 L/min, Applicants intend to recite thatthe flow rate may be about 1, about 2, about 3, about 4, about 5, about6, about 7, about 8, about 9, about 10, about 11, about 12, about 13,about 14, about 15, about 16, about 17, about 18, about 19, about 20,about 21, about 22, about 23, about 24, about 25, about 26, about 27,about 28, about 29, about 30, about 31, about 32, about 33, about 34,about 35, about 36, about 37, about 38, about 39, about 40, about 41,about 42, about 43, about 44, about 45, about 46, about 47, about 48,about 49, or about 50 L/min. Additionally, the flow rate may be withinany range from about 1 to about 50 L/min (for example, the rate is in arange from about 2 to about 20 L/min), and this also includes anycombination of ranges between about 1 and about 50 L/min. Likewise, allother ranges disclosed herein should be interpreted in a similar manner.

Embodiments of the present invention may provide systems, methods,and/or devices for the ultrasonic degassing of molten metals. Suchmolten metals may include, but are not limited to, aluminum, copper,steel, zinc, magnesium, and the like, or combinations of these and othermetals (e.g., alloys). Accordingly, the present invention is not limitedto any particular metal or metal alloy. The processing or casting ofarticles from a molten metal may require a bath containing the moltenmetal, and this bath of the molten metal may be maintained at elevatedtemperatures. For instance, molten copper may be maintained attemperatures of around 1100° C., while molten aluminum may be maintainedat temperatures of around 750° C.

As used herein, the terms “bath,” “molten metal bath,” and the like aremeant to encompass any container that might contain a molten metal,inclusive of vessel, crucible, trough, launder, and so forth. The bathand molten metal bath terms are used to encompass batch, continuous,semi-continuous, etc., operations and, for instance, where the moltenmetal is generally static (e.g., often associated with a crucible) andwhere the molten metal is generally in motion (e.g., often associatedwith a launder).

Many instruments or devices may be used to monitor, to test, or tomodify the conditions of the molten metal in the bath, as well as forthe final production or casting of the desired metal article. There is aneed for these instruments or devices to better withstand the elevatedtemperatures encountered in molten metal baths, beneficially having alonger lifetime and limited to no reactivity with the molten metal,whether the metal is (or the metal comprises) aluminum, or copper, orsteel, or zinc, or magnesium, and so forth.

Furthermore, molten metals may have one or more gasses dissolved inthem, and these gasses may negatively impact the final production andcasting of the desired metal article, and/or the resulting physicalproperties of the metal article itself. For instance, the gas dissolvedin the molten metal may comprise hydrogen, oxygen, nitrogen, sulfurdioxide, and the like, or combinations thereof. In some circumstances,it may be advantageous to remove the gas, or to reduce the amount of thegas in the molten metal. As an example, dissolved hydrogen may bedetrimental in the casting of aluminum (or copper, or other metal oralloy) and, therefore, the properties of finished articles produced fromaluminum (or copper, or other metal or alloy) may be improved byreducing the amount of entrained hydrogen in the molten bath of aluminum(or copper, or other metal or alloy). Dissolved hydrogen over 3-5 ppm,on a mass basis, may have detrimental effects on the casting rates andthe quality of resulting aluminum (or copper, or other metal or alloy)rods and other articles. Hydrogen may enter the molten aluminum (orcopper, or other metal or alloy) bath by its presence in the atmosphereabove the bath containing the molten aluminum (or copper, or other metalor alloy), or it may be present in aluminum (or copper, or other metalor alloy) feedstock starting material used in the molten aluminum (orcopper, or other metal or alloy) bath.

Attempts to reduce the amounts of dissolved gasses in molten metal bathshave not been completely successful. Often, these processes involveadditional and expensive equipment, as well as potentially hazardousmaterials. For instance, a process used in the metal casting industry toreduce the dissolved gas content of a molten metal may consist of rotorsmade of a material such as graphite, and these rotors may be placedwithin the molten metal bath. Chlorine gas additionally may be added tothe molten metal bath at positions adjacent to the rotors within themolten metal bath. This process will be referred to as the“conventional” process throughout this disclosure, and is often referredto in the industry as rotary gas purging. While the conventional processmay be successful in reducing, for example, the amount of dissolvedhydrogen in a molten metal bath in some situations, this conventionalprocess has noticeable drawbacks, not the least of which are cost,complexity, and the use of potentially hazardous and potentiallyenvironmentally harmful chlorine gas.

Additionally, molten metals may have impurities present in them, andthese impurities may negatively impact the final production and castingof the desired metal article, and/or the resulting physical propertiesof the metal article itself. For instance, the impurity in the moltenmetal may comprise an alkali metal or other metal that is neitherrequired nor desired to be present in the molten metal. As one of skillin the art would recognize, small percentages of certain metals arepresent in various metal alloys, and such metals would not be consideredto be impurities. As non-limiting examples, impurities may compriselithium, sodium, potassium, lead, and the like, or combinations thereof.Various impurities may enter a molten metal bath (aluminum, copper, orother metal or alloy) by their presence in the incoming metal feedstockstarting material used in the molten metal bath.

Embodiments of this invention may provide methods for reducing an amountof a dissolved gas in a molten metal bath or, in alternative language,methods for degassing molten metals. One such method may compriseoperating an ultrasonic device in the molten metal bath, and introducinga purging gas into the molten metal bath in close proximity to theultrasonic device. The dissolved gas may be or may comprise oxygen,hydrogen, sulfur dioxide, and the like, or combinations thereof. Forexample, the dissolved gas may be or may comprise hydrogen. The moltenmetal bath may comprise aluminum, copper, zinc, steel, magnesium, andthe like, or mixtures and/or combinations thereof (e.g., includingvarious alloys of aluminum, copper, zinc, steel, magnesium, etc.). Insome embodiments, the molten metal bath may comprise aluminum, while inother embodiments, the molten metal bath may comprise copper.Accordingly, the molten metal in the bath may be aluminum or,alternatively, the molten metal may be copper.

Moreover, embodiments of this invention may provide methods for reducingan amount of an impurity present in a molten metal bath or, inalternative language, methods for removing impurities. One such methodmay comprise operating an ultrasonic device in the molten metal bath,and introducing a purging gas into the molten metal bath in closeproximity to the ultrasonic device. The impurity may be or may compriselithium, sodium, potassium, lead, and the like, or combinations thereof.For example, the impurity may be or may comprise lithium or,alternatively, sodium. The molten metal bath may comprise aluminum,copper, zinc, steel, magnesium, and the like, or mixtures and/orcombinations thereof (e.g., including various alloys of aluminum,copper, zinc, steel, magnesium, etc.). In some embodiments, the moltenmetal bath may comprise aluminum, while in other embodiments, the moltenmetal bath may comprise copper. Accordingly, the molten metal in thebath may be aluminum or, alternatively, the molten metal may be copper.

The purging gas employed in the methods of degassing and/or methods ofremoving impurities disclosed herein may comprise one or more ofnitrogen, helium, neon, argon, krypton, and/or xenon, but is not limitedthereto. It is contemplated that any suitable gas may be used as apurging gas, provided that the gas does not appreciably react with, ordissolve in, the specific metal(s) in the molten metal bath.Additionally, mixtures or combinations of gases may be employed.According to some embodiments disclosed herein, the purging gas may beor may comprise an inert gas; alternatively, the purging gas may be ormay comprise a noble gas; alternatively, the purging gas may be or maycomprise helium, neon, argon, or combinations thereof; alternatively,the purging gas may be or may comprise helium; alternatively, thepurging gas may be or may comprise neon; or alternatively, the purginggas may be or may comprise argon. Additionally, Applicants contemplatethat, in some embodiments, the conventional degassing technique can beused in conjunction with ultrasonic degassing processes disclosedherein. Accordingly, the purging gas may further comprise chlorine gasin some embodiments.

However, in other embodiments of this invention, methods for degassingor for reducing an amount of a dissolved gas in a molten metal bath maybe conducted in the substantial absence of chlorine gas, or with nochlorine gas present. As used herein, a substantial absence means thatno more than 5% chlorine gas by weight may be used, based on the amountof purging gas used. In some embodiments, the methods disclosed hereinmay comprise introducing a purging gas, and this purging gas may beselected from the group consisting of nitrogen, helium, neon, argon,krypton, xenon, and combinations thereof.

The amount of the purging gas introduced into the bath of molten metalmay vary depending on a number of factors. Often, the amount of thepurging gas introduced in a method of degassing molten metals (and/or ina method of removing impurities from molten metals) in accordance withembodiments of this invention may fall within a range from about 0.1 toabout 150 standard liters/min (L/min). In some embodiments, the amountof the purging gas introduced may be in a range from about 0.5 to about100 L/min, from about 1 to about 100 L/min, from about 1 to about 50L/min, from about 1 to about 35 L/min, from about 1 to about 25 L/min,from about 1.5 to about 20 L/min, from about 2 to about 15 L/min, orfrom about 2 to about 10 L/min. These volumetric flow rates are instandard liters per minute, i.e., at a standard temperature (21.1° C.)and pressure (101 kPa).

In continuous or semi-continuous molten metal operations, the amount ofthe purging gas introduced into the bath of molten metal may vary basedon the molten metal output or production rate. Accordingly, the amountof the purging gas introduced in a method of degassing molten metals(and/or in a method of removing impurities from molten metals) inaccordance with such embodiments may fall within a range from about 10to about 500 mL/hr of purging gas per kg/hr of molten metal (mL purginggas/kg molten metal). In some embodiments, the ratio of the volumetricflow rate of the purging gas to the output rate of the molten metal maybe in a range from about 10 to about 400 mL/kg; alternatively, fromabout 15 to about 300 mL/kg; alternatively, from about 20 to about 250mL/kg; alternatively, from about 30 to about 200 mL/kg; alternatively,from about 40 to about 150 mL/kg; or alternatively, from about 50 toabout 125 mL/kg. As above, the volumetric flow rate of the purging gasis at a standard temperature (21.1° C.) and pressure (101 kPa).

Methods for degassing molten metals consistent with embodiments of thisinvention may be effective in removing greater than about 10 weightpercent of the dissolved gas present in the molten metal bath, i.e., theamount of dissolved gas in the molten metal bath may be reduced bygreater than about 10 weight percent from the amount of dissolved gaspresent before the degassing process was employed. In some embodiments,the amount of dissolved gas present may be reduced by greater than about15 weight percent, greater than about 20 weight percent, greater thanabout 25 weight percent, greater than about 35 weight percent, greaterthan about 50 weight percent, greater than about 75 weight percent, orgreater than about 80 weight percent, from the amount of dissolved gaspresent before the degassing method was employed. For instance, if thedissolved gas is hydrogen, levels of hydrogen in a molten bathcontaining aluminum or copper greater than about 3 ppm or 4 ppm or 5 ppm(on a mass basis) may be detrimental and, often, the hydrogen content inthe molten metal may be about 4 ppm, about 5 ppm, about 6 ppm, about 7ppm, about 8 ppm, about 9 ppm, about 10 ppm, about 15 ppm, about 20 ppm,or greater than 20 ppm. It is contemplated that employing the methodsdisclosed in embodiments of this invention may reduce the amount of thedissolved gas in the molten metal bath to less than about 4 ppm;alternatively, to less than about 3 ppm; alternatively, to less thanabout 2 ppm; alternatively, to within a range from about 1 to about 4ppm; alternatively, to within a range from about 1 to about 3 ppm; oralternatively, to within a range from about 2 to about 3 ppm. In theseand other embodiments, the dissolved gas may be or may comprisehydrogen, and the molten metal bath may be or may comprise aluminumand/or copper.

Embodiments of this invention directed to methods of degassing (e.g.,reducing the amount of a dissolved gas in bath comprising a moltenmetal) or to methods of removing impurities may comprise operating anultrasonic device in the molten metal bath. The ultrasonic device maycomprise an ultrasonic transducer and an elongated probe, and the probemay comprise a first end and a second end. The first end may be attachedto the ultrasonic transducer and the second end may comprise a tip, andthe tip of the elongated probe may comprise niobium. Specifics onillustrative and non-limiting examples of ultrasonic devices that may beemployed in the processes and methods disclosed herein will be discussedfurther below. As it pertains to an ultrasonic degassing process or to aprocess for removing impurities, the purging gas may be introduced intothe molten metal bath, for instance, at a location near the ultrasonicdevice. Often, the purging gas may be introduced into the molten metalbath at a location near the tip of the ultrasonic device. It iscontemplated that the purging gas may be introduced into the moltenmetal bath within about 1 meter of the tip of the ultrasonic device,such as, for example, within about 100 cm, within about 50 cm, withinabout 40 cm, within about 30 cm, within about 25 cm, or within about 20cm, of the tip of the ultrasonic device. In some embodiments, thepurging gas may be introduced into the molten metal bath within about 15cm of the tip of the ultrasonic device; alternatively, within about 10cm; alternatively, within about 8 cm; alternatively, within about 5 cm;alternatively, within about 3 cm; alternatively, within about 2 cm; oralternatively, within about 1 cm. In a particular embodiment, thepurging gas may be introduced into the molten metal bath adjacent to orthrough the tip of the ultrasonic device.

While not intending to be bound by this theory, Applicants believe thata synergistic effect may exist between the use of an ultrasonic deviceand the incorporation of a purging gas in close proximity, resulting ina dramatic reduction in the amount of a dissolved gas in a bathcontaining molten metal. Applicants believe that the ultrasonic energyproduced by the ultrasonic device may create cavitation bubbles in themelt, into which the dissolved gas may diffuse. However, Applicantsbelieve that, in the absence of the purging gas, many of the cavitationbubbles may collapse prior to reaching the surface of the bath of moltenmetal. Applicants believe that the purging gas may lessen the amount ofcavitation bubbles that collapse before reaching the surface, and/or mayincrease the size of the bubbles containing the dissolved gas, and/ormay increase the number of bubbles in the molten metal bath, and/or mayincrease the rate of transport of bubbles containing dissolved gas tothe surface of the molten metal bath. Regardless of the actualmechanism, Applicants believe that the use of an ultrasonic device incombination with a source of a purging gas in close proximity mayprovide a synergistic improvement in the removal of the dissolved gasfrom the molten metal bath, and a synergistic reduction in the amount ofdissolved gas in the molten metal. Again, while not wishing to be boundby theory, Applicants believe that the ultrasonic device may createcavitation bubbles within close proximity to the tip of the ultrasonicdevice. For instance, for an ultrasonic device having a tip with adiameter of about 2 to 5 cm, the cavitation bubbles may be within about15 cm, about 10 cm, about 5 cm, about 2 cm, or about 1 cm of the tip ofthe ultrasonic device before collapsing. If the purging gas is added ata distance that is too far from the tip of the ultrasonic device, thepurging gas may not be able to diffuse into the cavitation bubbles.Hence, while not being bound by theory, Applicants believe that it maybe beneficial for the purging gas to be introduced into the molten metalbath within about 25 cm or about 20 cm of the tip of the ultrasonicdevice, and more beneficially, within about 15 cm, within about 10 cm,within about 5 cm, within about 2 cm, or within about 1 cm, of the tipof the ultrasonic device.

Ultrasonic devices in accordance with embodiments of this invention maybe in contact with molten metals such as aluminum or copper, forexample, as disclosed in U.S. Patent Publication No. 2009/0224443, whichis incorporated herein by reference in its entirety. In an ultrasonicdevice for reducing dissolved gas content (e.g., hydrogen) in a moltenmetal, niobium or an alloy thereof may be used as a protective barrierfor the device when it is exposed to the molten metal, or as a componentof the device with direct exposure to the molten metal.

Embodiments of the present invention may provide systems and methods forincreasing the life of components directly in contact with moltenmetals. For example, embodiments of the invention may use niobium toreduce degradation of materials in contact with molten metals, resultingin significant quality improvements in end products. In other words,embodiments of the invention may increase the life of or preservematerials or components in contact with molten metals by using niobiumas a protective barrier. Niobium may have properties, for example itshigh melting point, that may help provide the aforementioned embodimentsof the invention. In addition, niobium also may form a protective oxidebarrier when exposed to temperatures of about 200° C. and above.

Moreover, embodiments of the invention may provide systems and methodsfor increasing the life of components directly in contact or interfacingwith molten metals. Because niobium has low reactivity with certainmolten metals, using niobium may prevent a substrate material fromdegrading. Consequently, embodiments of the invention may use niobium toreduce degradation of substrate materials resulting in significantquality improvements in end products. Accordingly, niobium inassociation with molten metals may combine niobium's high melting pointand its low reactivity with molten metals, such as aluminum and/orcopper.

In some embodiments, niobium or an alloy thereof may be used in anultrasonic device comprising an ultrasonic transducer and an elongatedprobe. The elongated probe may comprise a first end and a second end,wherein the first end may be attached to the ultrasonic transducer andthe second end may comprise a tip. In accordance with this embodiment,the tip of the elongated probe may comprise niobium (e.g., niobium or analloy thereof). The ultrasonic device may be used in an ultrasonicdegassing process, as discussed above. The ultrasonic transducer maygenerate ultrasonic waves, and the probe attached to the transducer maytransmit the ultrasonic waves into a bath comprising a molten metal,such as aluminum, copper, zinc, steel, magnesium, and the like, ormixtures and/or combinations thereof (e.g., including various alloys ofaluminum, copper, zinc, steel, magnesium, etc.).

FIG. 1 illustrates using niobium and other materials in an ultrasonicdevice 300, which may be used to reduce dissolved gas content in amolten metal. The ultrasonic device 300 may include an ultrasonictransducer 360, a booster 350 for increased output, and an ultrasonicprobe assembly 302 attached to the transducer 360. The ultrasonic probeassembly 302 may comprise an elongated ultrasonic probe 304 and anultrasonic medium 312. The ultrasonic device 300 and ultrasonic probe304 may be generally cylindrical in shape, but this is not arequirement. The ultrasonic probe 304 may comprise a first end and asecond end, wherein the first end comprises an ultrasonic probe shaft306 which is attached to the ultrasonic transducer 360. The ultrasonicprobe 304 and the ultrasonic probe shaft 306 may be constructed ofvarious materials. Exemplary materials may include, but are not limitedto, stainless steel, titanium, niobium, a ceramic (e.g., Sialon) and thelike, or combinations thereof. The second end of the ultrasonic probe304 may comprise an ultrasonic probe tip 310. The ultrasonic probe tip310 may comprise niobium. Alternatively, the tip 310 may consistentessentially of, or consist of, niobium. Niobium may be alloyed with oneor more other metals, or niobium may be a layer that is plated or coatedonto a base layer of another material. For instance, the tip 310 maycomprise an inner layer and an outer layer, wherein the inner layer maycomprise a ceramic or a metal material (e.g., titanium) and the outerlayer may comprise niobium. In this embodiment, the thickness of theouter layer comprising niobium may be less than about 25 microns, orless than about 10 microns, or alternatively, within a range from about2 to about 8 microns. For example, the thickness of the outer layercomprising niobium may be in range from about 3 to about 6 microns.

The ultrasonic probe shaft 306 and the ultrasonic probe tip 310 may bejoined by a connector 308. The connector 308 may represent a means forattaching the shaft 306 and the tip 310. For example the shaft 306 andthe tip 310 may be bolted or soldered together. In one embodiment, theconnector 308 may represent that the shaft 306 contains recessedthreading and the tip 310 may be screwed into the shaft 306. It iscontemplated that the ultrasonic probe shaft 306 and the ultrasonicprobe tip 310 may comprise different materials. For instance, theultrasonic probe shaft 306 may be or may comprise titanium and/orniobium, while the ultrasonic probe tip 310 may be or may compriseniobium. Alternatively, the ultrasonic probe shaft 306 may be or maycomprise titanium and/or a ceramic (e.g., Sialon), while the ultrasonicprobe tip 310 may be or may comprise a ceramic (e.g., Sialon).

In other embodiments, the ultrasonic probe 304 may be a single piece,e.g., the ultrasonic probe shaft 306 and the ultrasonic probe tip 310are a unitary part having the same construction. In such instances, theultrasonic probe may comprise, for instance, niobium or an alloythereof, a ceramic (e.g., Sialon), or other suitable material.

Referring again to FIG. 1, the ultrasonic device 300 may comprise aninner tube 328, a center tube 324, an outer tube 320, and a protectiontube 340. These tubes or channels may surround at least a portion of theultrasonic probe 304 and generally may be constructed of any suitablemetal or ceramic material. It may be expected that the ultrasonic probetip 310 will be placed into the bath of molten metal; however, it iscontemplated that a portion of the protection tube 340 also may beimmersed in molten metal. Accordingly, the protection tube 340 may be ormay comprise titanium, niobium, silicon carbide, a ceramic, or acombination of more than one of these materials. Contained within thetubes 328, 324, 320, and 340 may be fluids 322, 326, and 342, asillustrated in FIG. 1. The fluid may be a liquid or a gas (e.g., argon),the purpose of which may be to provide cooling to the ultrasonic device300 and, in particular, to the ultrasonic probe tip 310 and theprotection tube 340.

The ultrasonic device 300 may comprise an end cap 344. The end cap maybridge the gap between the protection tube 340 and the probe tip 310 andmay reduce or prevent molten metal from entering the ultrasonic device300. Similar to the protection tube 340, the end cap 344 may be or maycomprise, for example, titanium, niobium, silicon carbide, a ceramic, ora combination of more than one of these materials.

The ultrasonic probe tip 310, the protection tube 340, or the end cap344, or all three, may comprise niobium. Niobium alone may be used,niobium may be alloyed with one or more other metals, or niobium may bea layer that is plated or coated onto a base layer of another material.For instance, the ultrasonic probe tip 310, the protection tube 340, orthe end cap 344, or all three, may comprise an inner layer and an outerlayer, wherein the inner layer may comprise a ceramic or a metalmaterial and the outer layer may comprise niobium. It may be expectedthat the presence of niobium on parts of the ultrasonic device mayimprove the life of the device, may provide low or no chemicalreactivity when in contact with molten metals, may provide strength atthe melting temperature of the molten metal, and may have the capabilityto propagate ultrasonic waves. In accordance with some embodiments ofthis invention, when the tip 310 of the ultrasonic device does notcomprise niobium, the tip may show erosion or degradation after onlyabout 15-30 minutes in a molten metal bath (e.g., of aluminum orcopper). In contrast, when the tip of the ultrasonic device comprisesniobium, the tip may show no or minimal erosion or degradation after atleast 1 hour or more, for instance, no erosion or degradation after atleast 2 hours, after at least 3 hours, after at least 4 hours, after atleast 5 hours, after at least 6 hours, after at least 12 hours, after atleast 24 hours, after at least 48 hours, or after at least 72 hours.

In another embodiment, the ultrasonic probe tip 310, the protection tube340, or the end cap 344, or all three, may comprise a ceramic, such asSialon. Further, the ultrasonic probe shaft 306 may comprise a ceramic,or alternatively, titanium.

FIG. 2 illustrates another ultrasonic device 400 that may compriseniobium, a ceramic such as Sialon, or other suitable material. Theultrasonic device 400 may include an ultrasonic transducer 460, abooster 450 for increased output, and an ultrasonic probe assembly 402attached to the transducer 460. The booster 450 may permit increasedoutput at boost levels greater than about 1:1, for instance, from about1.2:1 to about 10:1, or from about 1.4:1 to about 5:1. A booster clampassembly 451 having a height H may be employed, where the height H mayvary as needed to accommodate different length ultrasonic probes. Theultrasonic probe assembly 402 may comprise an elongated ultrasonic probeas depicted in FIG. 1 and an ultrasonic probe tip 410. The ultrasonicprobe and tip may be constructed of various materials, as previouslydiscussed, including, but not limited to, stainless steel, titanium,niobium, ceramics, and the like, or combinations thereof, inclusive ofmixtures thereof, alloys thereof, and coatings thereof.

The ultrasonic device 400 may comprise a means for introducing a purginggas (e.g., into a molten metal bath) at a location near the ultrasonicdevice 400. It is contemplated that an external purging gas injectionsystem (not shown) may be positioned in the molten metal bath, and theinjection site may be near the ultrasonic device of FIG. 1 and/or FIG.2. Alternatively, the ultrasonic device may comprise a purging gasoutlet, such that the purging gas may be expelled near or at the tip ofthe ultrasonic device. For instance, the purging gas may be expelledthrough the end cap of the ultrasonic device and/or through the probe ofthe ultrasonic device. Referring again to FIG. 2, the ultrasonic devicemay comprise a purging gas inlet port 424 and injection chamber 425,connected to a purging gas delivery channel 413. The purging gas may bedelivered to, and expelled through, a purging gas delivery space 414located near or at the tip 410 of the ultrasonic device 400. It iscontemplated that the purging gas delivery space 414, or purging gasoutlet, may be within about 10 cm of the tip 410 of the ultrasonicdevice 400, such as, for example, within about 5 cm, within about 3 cm,within about 2 cm, within about 1.5 cm, within about 1 cm, or withinabout 0.5 cm, of the tip of the ultrasonic device.

Additionally, the ultrasonic device 400 may comprise an ultrasoniccooler system 429, which may be designed to keep the ultrasonic tipand/or the ultrasonic probe and/or the ultrasonic probe assembly at atemperature closer to room temperature (e.g., the temperature may be ina range from about 15° C. to about 75° C., or from about 20° C. to about35° C.), as opposed to the elevated temperatures of molten metalexperienced by the outer surface of the tip 410 of the ultrasonicdevice. It is contemplated that an ultrasonic cooler system may not berequired if the ultrasonic probe and assembly comprise niobium, aceramic such as Sialon, or other suitable material. The ultrasoniccooler system 429 of FIG. 2 may be similar to that system depicted inFIG. 1 including, for instance, an inner tube 328, a center tube 324, anouter tube 320, a protection tube 340, and fluids 322, 326, and 342,designed to provide cooling and/or temperature control to the ultrasonicdevice. The fluid may be a liquid or a gas, and it is contemplated thatthe fluid may be the same material as the purging gas.

FIG. 3 illustrates yet another ultrasonic device 500 that may compriseniobium, a ceramic such as Sialon, or other suitable material. Theultrasonic device 500 may include an ultrasonic transducer 560, abooster 550 for increased output, and an ultrasonic probe assembly 510attached to the transducer 560. The booster 550 may permit increasedoutput at boost levels greater than about 1:1, for instance, from about1.2:1 to about 10:1, or from about 1.4:1 to about 5:1. The ultrasonicprobe 510 may be a single piece, or the ultrasonic probe 510 maycomprise an ultrasonic probe shaft and an optional (and replaceable)ultrasonic probe tip 511, similar to that depicted in FIG. 1. Theultrasonic probe and tip may be constructed of various materials, aspreviously discussed, including, but not limited to, stainless steel,titanium, niobium, ceramics, and the like, or combinations thereof,inclusive of mixtures thereof, alloys thereof, and coatings thereof.

The ultrasonic device 500 may comprise a means for introducing a purginggas (e.g., into a molten metal bath) at a location near the ultrasonicdevice 500 and/or near the ultrasonic probe tip 511. As above, It iscontemplated that an external purging gas injection system (not shown)may be positioned in the molten metal bath, and the injection site maybe near the ultrasonic device of FIG. 3. Alternatively, the ultrasonicdevice may comprise a purging gas outlet, such that the purging gas maybe expelled near or at the tip of the ultrasonic device. For instance,the purging gas may be expelled through the probe/tip of the ultrasonicdevice. Referring again to FIG. 3, the ultrasonic device may comprise apurging gas inlet port 522 in a chamber with the booster 550, an upperhousing 520, lower support housing 521, and a lower support housingcover 523. The upper housing 520 may be gas tight and/or leak proof. Thepurging gas inlet port 522 may be connected to a purging gas deliverychannel 524, which may be contained within the ultrasonic probe 510. Thepurging gas may be delivered to, and expelled through, a purging gasinjection point 525 located at the tip 511 of the ultrasonic device 500.Accordingly, in this embodiment, the ultrasonic device 500 may comprisean ultrasonic probe 510 comprising a purging gas injection system with apurging gas injection point at the tip of the ultrasonic probe.

Optionally, the ultrasonic device 500 may comprise an ultrasonic coolersystem, such as described above relative to FIG. 1 and/or FIG. 2, butthis is not a requirement.

While certain embodiments of the invention have been described, otherembodiments may exist. Further, any disclosed methods' stages may bemodified in any manner, including by reordering stages and/or insertingor deleting stages, without departing from the invention. While thespecification includes examples, the invention's scope is indicated bythe following claims. Furthermore, while the specification has beendescribed in language specific to structural features and/ormethodological acts, the claims are not limited to the features or actsdescribed above. Rather, the specific features and acts described aboveare disclosed as illustrative embodiments of the invention.

EXAMPLES Examples 1-4

In Examples 1-4, a series of tests were conducted to demonstrate thereduction in the amount of dissolved hydrogen in a molten bath ofaluminum that may be achieved by the disclosed methods. A control sampleof the aluminum was taken and tested prior to the use of any degassingtechnique (Example 1). The molten metal bath of aluminum was operatingat a temperature of about 1350° F. (732° C.). A conventional degassingtechnique, rotary gas purging, was then employed to determine theeffectiveness of conventional methods of hydrogen removal (Example 2).Example 3 utilized an ultrasonic degassing process as disclosed herein,namely, an ultrasonic device in combination with the introduction of apurging gas. In Example 3, the ultrasonic device contained a niobiumtip, and the tip of the ultrasonic device was placed into the aluminumbath. The ultrasonic device was operated at 20,000 Hz (frequency) in themolten bath of aluminum. Concurrently with the operation of theultrasonic device, the purging gas argon was introduced into the moltenmetal bath at a rate of about 4.7 standard liters per minute (L/min).The argon was injected along the tip of the ultrasonic device (thedistance between the injection point and the tip was less than about 2cm). Example 4 utilized both the ultrasonic degassing process incombination with the conventional degassing technique.

Aluminum samples of Example 1 (no degassing), Example 2 (afterconventional degassing), Example 3 (after ultrasonic degassing), andExample 4 (after ultrasonic and conventional degassing) were allowed tocool and solidify under vacuum. Then, one cubic centimeter (1 cc=1 mL)cubes from each example were measured to determine the mass and,accordingly, the density of the aluminum of each example. Aluminum has atheoretical density of 2.7 g/cc, and the presence of hydrogen gas inaluminum will reduce this density. FIG. 4 shows the percentagedifference in density for each of Examples 1-4 as compared to thetheoretical density of aluminum. In FIG. 4, the closer to thetheoretical density of aluminum that each sample is (i.e., the lower thepercentage below the density of aluminum), the more effective thedegassing procedure. As demonstrated in FIG. 4, the ultrasonic procedure(Example 3) was as effective as the conventional technique (Example 2),and the use of both in combination (Example 4) may offer a slightadditional improvement.

Aluminum samples of Examples 1-4 were also evaluated for the ppmhydrogen content (on a mass basis). Cast samples that were cooled andsolidified under vacuum were analyzed for hydrogen content. The hydrogencontent analyses are summarized in FIG. 5. In FIG. 5, the lower thehydrogen content in ppm, the more effective the degassing procedure. Asdemonstrated in FIG. 5, the ultrasonic procedure (Example 3) was moreeffective in removing hydrogen than the conventional technique (Example2), and the use of both in combination (Example 4) did not appear tooffer any additional benefit. The data of FIG. 5 are no longer reliedupon. Applicants believe there was an analytical error in thedetermination of the listed ppm hydrogen content.

Examples 5-8

In Examples 5-8, a series of tests were conducted to determine therelative speed at which dissolved hydrogen in a molten bath of aluminumcan be degassed in accordance with the disclosed methods. First, a smallamount of aluminum was melted in a metal bath, and then maintained, at atemperature of about 1350° F. (732° C.). An Alspek unit was used todetermine a baseline reading of hydrogen content, in units of mL/100 g.The Alspek unit uses the principle of partial pressures in anelectrolytic half cell to determine the amount of dissolved hydrogen inmolten aluminum. The tip of an ultrasonic device was placed into thealuminum bath, and the purging gas argon was added to the molten metalbath at a rate of about 1 standard liter per minute (L/min). ForExamples 5-7, the ultrasonic device was operated with a 3:1 booster andat 20,000 Hz, although up to and including 40,000 Hz, or more, could beused. For Example 5, a baseline ultrasonic vibration amplitude was used,and a baseline power level for the ultrasonic power supply (watts); forExample 6, the ultrasonic vibration amplitude was 2 times the baseline,and the power level of the ultrasonic power supply was 1.9 times thebaseline; and for Example 7, the ultrasonic vibration amplitude was 3times the baseline, and the power level of the ultrasonic power supplywas 3.6 times the baseline. For Example 8, the ultrasonic device was notused, only addition of the argon purging gas. The level of hydrogen wasmonitored over time using the Alspek unit, and recorded. Between eachexperiment, hydrogen was added into the aluminum bath, and the baselinebefore the addition of the argon gas was determined.

An ultrasonic device similar to that illustrated in FIG. 3 was used inExamples 5-8. The ultrasonic device did not have a cooling assembly, andthe purging gas was injected thru the tip of the ultrasonic probe. Theultrasonic probe was 1″ (2.5 cm) in diameter, and both the probe and tip(as a single part) were constructed of a niobium alloy containinghafnium and titanium.

FIG. 6 illustrates a plot of hydrogen concentration in mL of hydrogenper 100 g of the aluminum alloy as a function of time after the additionof the argon purging gas (and the activation of the ultrasonic device,if used). FIG. 6 demonstrates the each of Examples 5-7 degassed hydrogenfrom aluminum significantly faster (using a purging gas and anultrasonic device) than that of Example 8, which only used a purginggas, but no ultrasonic device. Examples 6-7 performed slightly betterthan Example 5, which used a lower ultrasonic vibration amplitude and alower baseline power level for the ultrasonic power supply.

Examples 9-10

Examples 9-10 were large scale trials to determine the effectiveness ofusing a purging gas and an ultrasonic device to remove hydrogen andlithium/sodium impurities in a continuous casting experiment usingaluminum alloy 5154 (containing magnesium). The temperature of the metalbath was maintained at a temperature of about 1350° F. (732° C.).

Sodium and lithium concentrations in weight percent were determinedusing a spectrometer, and hydrogen concentrations were determined usingan Alscan hydrogen analyzer for molten aluminum. Example 9 was a controlexperiment, and the prevailing sodium and lithium concentrations in themolten aluminum alloy of Example 9 were 0.00083% (8.3 ppm) and 0.00036%(3.6 ppm), respectively. The hydrogen concentration in Example 9 was0.41 mL/100 g.

The ultrasonic device of Examples 5-8 was used in Example 10 andoperated at 20,000 Hz. In conjunction with the operation of theultrasonic device, in Example 10, argon gas was added to the moltenmetal bath at a volumetric flow rate of about 80-85 mL/hr per kg/hr ofmolten metal output (i.e., 80-85 mL purging gas/kg molten metal). Afterthe use of the ultrasonic device and the argon purging gas, the sodiumconcentration in the molten aluminum alloy was below the minimumdetection limit of 0.0001% (1 ppm), and the lithium concentration in themolten aluminum alloy was 0.0003% (3 ppm). The hydrogen concentration inExample 10 was 0.35 mL/100 g, a reduction of about 15%.

What is claimed is:
 1. A method for reducing an amount of a dissolvedgas in a molten metal bath, the method comprising: operating anultrasonic device in the molten metal bath, the ultrasonic devicecomprising: an ultrasonic transducer, an elongated probe comprising afirst end and a second end, the first end attached to the ultrasonictransducer and the second end comprising a tip, and a purging gasdelivery system, the purging gas delivery system comprising a purginggas inlet and a purging gas outlet, wherein the elongated probecomprises a ceramic, and the purging gas outlet is at the tip of theelongated probe; and introducing a purging gas into the molten metalbath through the purging gas outlet at the tip of the elongated probe.2. The method of claim 1, wherein the dissolved gas comprises oxygen,hydrogen, sulfur dioxide, or a combination thereof.
 3. The method ofclaim 1, wherein the molten metal bath comprises aluminum, copper, zinc,steel, magnesium, or a combination thereof.
 4. The method of claim 1,wherein the purging gas comprises nitrogen, helium, neon, argon,krypton, xenon, or combinations thereof.
 5. The method of claim 1,wherein the purging gas is introduced into the molten metal bath at arate in a range from about 0.1 to about 150 L/min.
 6. The method ofclaim 1, wherein the purging gas is introduced into the molten metalbath at a rate in a range from about 10 to about 500 mL/hr of purginggas per kg/hr of output from the molten metal bath.
 7. The method ofclaim 1, wherein the method results in a reduction of at least about 10weight percent in the amount of the dissolved gas in the molten metalbath.
 8. The method of claim 1, wherein the method results in areduction of alkali metal impurities in the molten metal bath.
 9. Themethod of claim 1, wherein the dissolved gas comprises hydrogen, andwherein the method results in a reduction of the amount of hydrogen inthe molten metal bath to less than about 4 ppm.
 10. The method of claim1, wherein the method is conducted in the substantial absence ofchlorine gas.
 11. The method of claim 1, wherein: the purging gas isintroduced into the molten metal bath at a rate in a range from about 1to about 50 L/min; the dissolved gas comprises hydrogen; the moltenmetal bath comprises aluminum, copper, or a combination thereof; and thepurging gas comprises argon, nitrogen, or a combination thereof.
 12. Themethod of claim 11, wherein the purging gas is introduced into themolten metal bath at a rate in a range from about 30 to about 200 mL/hrof purging gas per kg/hr of output from the molten metal bath.
 13. Themethod of claim 1, wherein the elongated probe comprises Sialon.
 14. Themethod of claim 1, wherein the ultrasonic device further comprises acooling system surrounding at least a portion of the elongated probe.15. The method of claim 14, wherein the cooling system comprises atleast one channel surrounding at least a portion of the elongated probe.16. The method of claim 15, wherein the channel comprises a fluid. 17.The method of claim 16, wherein the fluid is argon.